The present invention relates to valves and, more particularly, to steam valves such as used, for example, in steam turbines for controlling the flow of steam from a steam chest to the interior of the steam turbine.
As is well known, the forces required to open a steam valve vary markedly as the steam valve moves between a closed position and an open position. This results from the dramatic change in differential pressure across the valve during the opening and closing procedure That is, when the valve is fully closed and sealed, the full steam pressure acts across the valve area in the direction tending to maintain the valve in the closed position. Countering forces from beneath the valve are minimal at this time. When the valve is fully opened, the steam pressure beneath the valve is about the same as that above the valve, and thus very little force is required to move the valve in this region.
Of particular criticality is operating the valve in the vicinity of its cracking position. From a fully closed position to a very small opening position (cracking position), the valve forces go through a very large change.
In the prior art, the large forces required to crack a steam valve are overcome using an auxiliary piston in a balance chamber. The auxiliary piston is exposed to a steam pressure related to the steam pressure to which the valve is subjected. The piston in the balance chamber exerts forces on the valve in the opening direction, whereby the valve actuator is required to provide only the difference between the closing force exerted on the valve and the opening force exerted on the auxiliary piston in the balance chamber.
Smaller required actuating forces permit the use of smaller actuating devices such as, for example, hydraulic cylinders, handwheels, and the like. This has the desirable result of reduced size and cost.
Minimum actuation forces are achieved when the opening and the closing forces are balanced. However, if the initial cracking force is made about zero by near-perfect balance, as soon as the valve plug is moved a short distance from its seat, the large change in valve force tends to apply an opening force to the valve.
The valve travel distance to achieve dramatic changes in valve actuating force in the vicinity of cracking is often in the range of clearances, tolerances and deflections in the valve construction. Perfect balance in the valve when closed would result in a seriously out-of-balance condition in the cracked position The resulting change in force vectors can cause the valve plug to flutter or vibrate, thus leading to unstable control, noise, and rapid wear of the parts.
Thus, stable valve operation requires a substantial imbalance in the closing direction of the closed valve, such that an imbalance in the closing direction remains during, and beyond cracking. This requires an actuator capable of exerting a substantial force to overcome the imbalance. Substantially beyond the cracking position, of course, the force requirements are reduced substantially. In the prior art, however, the same actuating force is available, but is not required, once the valve is beyond its cracking position.
A further prior-art technique employs a pilot valve associated with a main steam valve. The pilot valve is opened first, then the main steam valve is opened. This two-step procedure at least partly alleviates the problem of the large changes in actuating force taking place in the vicinity of the closed position.